aeons

An aeon (plural aeons) is a term for a very long period of time. In geology, an eon is the largest formal division of the geologic time scale, spanning hundreds of millions to billions of years and containing several eras. The current eon is the Phanerozoic, which began about 541 million years ago and continues to the present. The Phanerozoic is subdivided into three eras: Paleozoic, Mesozoic, and Cenozoic. The preceding eons—Hadean, Archean, and Proterozoic—constitute the Precambrian, spanning from Earth's formation to the start of the Phanerozoic.

As a broader concept, 'aeon' is used to denote an immense, often indefinite span of time. In cosmology and popular science, it can be used interchangeably with 'epoch' or 'era' and, in some contexts, to denote a billion years (a gigayear). In religious and philosophical traditions, aeons may refer to divine emanations or ages; in Gnostic cosmology, aeons are emanations of the divine realm that populate the pleroma.

Spelling varies by variant of English: 'aeon' is common in British English, while 'eon' is typical in American usage. The term is distinct from 'era,' which is a smaller unit in the geologic time scale, and from 'epoch' or 'age,' which are progressively shorter divisions. See also geologic timescale, cosmology, and Gnosticism.